Filtration media have been used for the filtration of fine particles from fluids, particularly liquids, for many years. Such filtration media are available in a variety of materials to meet particular filtration requirements. Microporous membranes, such as those described in U.S. Pat. No. 4,340,479, are particularly well-suited to the filtration of fine particulate matter from fluids.
Many filtration media, such as microporous membranes, possess the ability to remove fine particles but unfortunately suffer from a lack of mechanical strength, e.g., they are relatively fragile. As a result, a support material is often mated with such a filtration medium in order to provide the filtration medium with an adequate degree of mechanical support. This is particularly the case when the filtration medium is used in high shear or pulsed flow environments or is subjected to high backflow pressures.
There have been many attempts at mating filtration media, particularly microporous membranes, to suitable support materials. These attempts have included preparing a filtration medium directly onto a support material, thermally laminating a filtration medium directly onto a support material, and utilizing an adhesive to secure a filtration medium to a support material. Each of these techniques is not without problems, such as weak filtration medium-support adherence, significant blockage of the pores of the filtration medium, alteration of the physical characteristics of the filtration medium, and the introduction of possible contamination sources.
Supported filtration assemblies have been produced by thermally bonding a nonwoven mat of melt-blown fibers to a film. This method involves either pressing a nonwoven mat of fibers against a heated film, preferably as the film is being produced, or melt-blowing the fibers directly onto the film, which may be at ambient or an elevated temperature. This method, however, has general applicability to nonwoven mats and melt-extruded films of the same material, rather than to microporous filtration membranes which could be easily distorted or clogged by this method.
Supported membrane assemblies may also be produced by contemporaneously forming and integrally securing a membrane to the surface of a substrate. This method, however, is severely limited by the requirement that the membrane be precipitated from a liquid suspension and secured to the substrate in a single step. Such porous media, moreover, may delaminate in reverse flow at low pressures, often at differential pressures below about 70 kPa. Moreover, some membranes, which may be employed effectively in filter applications, are not formed from liquid suspension. For example, polytetrafluoroethylene (PTFE) is typically made as a powder, which is then extruded to form a sheet, and the sheet is biaxially stretched to form a porous membrane.
A membrane may also be secured to a substrate by a method which involves the application of a solvent to which the membrane is inert, but which dissolves the support material. The membrane is saturated with the solvent, and then contacted with the support material. The contact of the saturated membrane with the support material dissolves a portion of the support material, which is then integrally secured to the membrane after the solvent is removed. This method has the severe fault that it can be extremely difficult to maintain a uniform distribution of solvent throughout the membrane at the time it is applied to the support material. Simple dipping, or any procedure involving manipulation of the wet membrane, invariably leaves more solvent in some portions of the membrane than in others. As a result, an excessively thick bond may form in some areas of contact, while in other areas the bonding between the membrane and the support material may be inadequate. Also, manipulation of the membrane is made more difficult by the rapid evaporation of the solvent, such that a significant loss of solvent can occur in a few seconds, thereby further complicating the effort of obtaining a uniformly secure bond. Further, as the solvent evaporates during the dissolution and bonding process, there may be migration of dissolved support material into the pores of the membrane such that dissolved support material may be deposited within the membrane, thereby at least partially clogging (i.e., altering the pore size and decreasing the permeability of) the membrane.
Thus, there remains a need for a method for adhering a membrane to the surface of a support material, particularly a rigid support material, which method provides secure adherence of the membrane to the support material without substantially adversely affecting either the membrane or the support material. The present invention seeks to provide such a method and the resulting supported membrane assembly. These and other objects and advantages of the present invention, as well as additional inventive features, will be apparent from the description of the invention set forth herein.